1. Field of the Invention
The present invention relates to a method of manufacturing a nano-scale LED electrode assembly including a selective metal ohmic layer, and more particularly, to a method of manufacturing a nano-scale LED electrode assembly including a selective metal ohmic layer, which is capable of increasing conductivity between electrodes and a nano-scale LED device and also reducing contact resistance therebetween.
2. Discussion of Related Art
The development of light emitting diodes (LEDs) has been facilitated since 1992 when Nakamura et al. from Nichia Chemical Corporation (Japan) succeeded in fusing a high-quality single-crystal gallium nitride (GaN) semiconductor by applying a low-temperature GaN compound buffer layer. An LED is a semiconductor device having a structure in which n-type semiconductor crystals having a plurality of carriers, i.e., electrons, and p-type semiconductor crystals having a plurality of carriers, i.e., holes, are joined using the characteristics of a compound semiconductor, that is, a semiconductor device that converts an electrical signal into light having a wavelength band at a desired region to emit the light.
Such an LED semiconductor is called a revolution of light as a green material because the LED semiconductor has very low energy consumption due to high light conversion efficiency, has a semi-permanent lifespan and is environmentally friendly. With the current development of compound semiconductor technology, red, orange, green, blue, and white LEDs having high brightness have been developed. Also, the LEDs have been applied to various fields such as traffic lights, mobile phones, headlights for vehicles, outdoor electronic display boards, LED backlight units (BLUs), and indoor/outdoor lightings, and thus research on the LEDs is still being actively conducted. In particular, a GaN-based compound semiconductor material having a wide band gap is a material that has been used to manufacture LED semiconductors which emit green and blue light and ultraviolet rays. Since it is possible to manufacture white LED devices using blue LED devices, much research on such a manufacturing method has been conducted.
Owing to the application of the LED semiconductor to various fields and the research on the LED semiconductor, LED semiconductors having high power output are also needed, and thus it is very important to improve efficiency of the LED semiconductor. However, there are several difficulties in manufacturing the blue LED device having high efficiency and high power output.
The difficulties in improving the efficiency of the blue LED device may result from the difficulty in a manufacturing process and a high refractive index of the manufactured LED between the GaN-based semiconductor and the atmosphere.
First, the difficulty in the manufacturing process may be due to the difficulty in manufacturing a substrate having the same lattice constant as the GaN-based semiconductor. A GaN epitaxial layer formed on the substrate has a drawback in that many defects may occur when the lattice constant of the epitaxial layer is significantly mismatched with that of the substrate, resulting in lowered efficiency and performance.
Next, light emitted from an active layer of the LED does not escape to the outside but is totally reflected to the inside of the LED due to the high refractive index of the manufactured blue LED between the GaN-based semiconductor and the atmosphere. Such totally reflected light may be reabsorbed inside of the LED, resulting in deteriorated efficiency of the LED. Such efficiency may be referred to light extraction efficiency of an LED device. To solve the above problems, much research is being conducted.
Meanwhile, to make use of the LED device in lightings, displays, and the like, an LED device and an electrode for applying power to the LED device are required. Also, research on an arrangement of the LED device and two different electrodes in connection with an application purpose, a decrease in a space occupied by the electrodes, or a manufacturing method has been variously conducted.
Research on the arrangement of the LED device and the electrodes may be classified into a method of growing an LED device on electrodes and a method of separately growing an LED device and arranging the LED device on electrodes.
First, in research on growing the LED device on the electrodes, there is a bottom-up method in which the LED device and the electrodes are formed and arranged at the same time through a series of manufacturing processes using a method which includes forming a lower electrode on a substrate in the form of a thin film, and sequentially stacking an n-type semiconductor layer, an active layer, a p-type semiconductor layer, and an upper electrode on the lower electrode and etching the stacked layers, or etching the previously stacked layers prior to stacking the upper electrode and then stacking the upper electrode.
Next, the method of separately and independently growing an LED device and arranging the LED device on electrodes is a method including independently growing LED devices using a separate process, and then arranging the manufactured LED devices on patterned electrodes one by one.
The former method has a problem in that it is very difficult to grow a high-crystalline/high-efficiency thin film and an LED device in a crystallographic aspect, and the latter has a problem in that light emitting efficiency may be deteriorated due to low light extraction efficiency.
Also, the latter method has a problem in that a three-dimensional (3D) LED device may be erected and connected to electrodes in case of the conventional LED devices, but it is very difficult to erect a 3D LED device on electrodes when the LED device is an LED device having a nano-scale size. Korean Patent Application No. 2011-0040174 filed by the inventor of this application discloses a coupling linker for promoting coupling of a nano-scale LED device having a nano-scale size to electrodes in a state in which the LED device is coupled to the electrodes in a 3D erect state. In fact, however, it is very difficult to couple the nano-scale LED device to the electrodes in the 3D erect state when utilizing nano-scale electrodes.
Further, the separately manufactured LED devices have to be arranged one by one on the patterned electrodes. However, when the size of the LED devices is very small (e.g., a nano-scale size), the LED devices have drawbacks in that it is very difficult to arrange the LED devices on two different nano-scale electrodes within a desired range, and defects often occur due to electrical short circuits between the electrodes and the nano-scale LEDs even when the LED devices are disposed on the two different nano-scale electrodes, which makes it impossible to realize a desired electrode assembly.
Also, Korean Patent Application No. 2010-0042321 discloses a structure of an address electrode line for LED modules and a method of manufacturing the same. In case of this application, a lower electrode is formed on a substrate in the form of a thin film, and an insulation layer and an upper electrode are sequentially stacked on the lower electrode, and then etched to manufacture an electrode line. Then, an LED chip is mounted on the upper electrode. However, when the mounted LED chip has a nano-scale size, it is very difficult to accurately mount the 3D LED chip on the upper electrode in an erect state. Even when the LED chip is mounted on the upper electrode, it is also difficult to connect the mounted LED chip having the nano size to the lower electrode.
Further, when the separately mounted LED devices are disposed on the electrodes to apply power to the electrodes, contact resistance between the LED devices and the electrodes occurs, resulting in degraded light extraction efficiency.